Dr. Bloedorn is a health psychologist who researches nutrition. She is curious as to whether a new drink additive will help peop

le consume fewer calories during a meal. The drink additive is a white, odorless, tasteless powder that a person can add to any drink. She plans to collect a random sample of 50 overweight students on campus and measure the calories they eat during lunch, using a calorimeter. She plans to give this additive to the same participants to use at lunch a few days later and measure how many calories they eat (again, using the calorimeter).
Which of the following threats to internal validity will Dr. Bloedorn NOT be worried about?
a. Instrumentation
b. Attritionc. Historyd. Placebo effects